In the measurement of caliper of paper, caliper gauges have been of both the contacting and non-contacting types. Contacting gauges are perhaps more reliable but because they bear against the sheet material (which is perhaps fluttering and shifting) may cause ripping or other unwanted effects. On the other hand, the non-contacting gauge which, for example, might utilize air jets to space the gauge from the surface of the moving paper, is less reliable and in general, less accurate.
Both contacting and non-contacting gauges use the general concept of measurement of the reluctance of a magnetic flux path through pole pieces of the gauge which is made of magnetic material, with the gap between the gauge varying according to caliper of the paper. This also changes the inductance of the circuit and when such inductance is placed in a resonant circuit with a capacitor, the resonant frequency is thus indicative of variation in the gap, which is proportional to the caliper of the paper.
In addition to the above difficulties in making a choice between a contacting and non-contacting gauge with the disadvantages of each, in general the inductive technique of measurement may not provide satisfactory sensitivity at some paper thicknesses.